Generally, as a method of measuring total nitrogen in wastewater discharged from factories and the like, “Ultraviolet spectrophotometry” defined in “Testing methods for industrial wastewater discharged from factories” according to Japanese Industrial Standards is used. This is a method of adding alkaline potassium peroxodisulfate, which is an oxidizing agent, to a sample water, decomposing nitrogen compounds into nitrate ions in a high-temperature and high-pressure environment by thermal decomposition, and determining total nitrogen concentration by absorbance measurement at 220 nm. However, a measurement apparatus to be used for this method has to be resistant to pressure and heat, thus requiring special materials and design.
As another method of decomposing nitrogen compounds in a sample water into nitrate ions by oxidation, there is an “ultraviolet oxidative decomposition method”. According to this method, total nitrogen concentration is determined by decomposing nitrogen compounds in a sample water to which alkaline potassium peroxodisulfate, which is an oxidizing agent, is added into nitrate ions by oxidation by radiating ultraviolet rays on the sample water under a condition of predetermined temperature and pressure, and measuring the absorbance of the sample water at 220 nm. This method is improved in reducing the pressure and temperature for the sample water, and the temperature necessary for oxidative decomposition of nitrogen oxide by ultraviolet rays is reduced to about 60° C. from conventional 120° C., and also, the pressure may be a normal pressure, and thus, the apparatus is not required to be highly resistant to pressure and heat.